In combustion chambers for gas turbines, it is an aim to reduce the emissions, such as nitrogen oxides NOx and/or carbon monoxide CO. The temperatures inside the combustion chamber cause high CO and NOx emissions.
In order to reduce the emissions, so-called dry low emissions (DLE) combustion systems are used which typically have a main and supplementary fuel stream, where the main fuel stream burns in a premixed flame mode. The supplementary fuel stream may generate a so-called pilot flame, which may be of a pure diffusion type or to some extent premixed. This stabilizes a main flame in the combustion chamber. A very lean fuel mixture is burned in the main flame. Under normal circumstances this would cause an unstable flame which is prone to dynamics. Therefore, the DLE systems use the pilot flame. The pilot flame comprises a rich or richer fuel mixture, wherein the rich or richer pilot flame is more stable than the lean main flame and the heat and radicals produced from this hot stable pilot frame stabilizes the main flame.
Ignition of liquid system in gas turbines has always proved problematic. There are many factors involved, such as the fuel flow rate, atomization of the fuel, air assistant flow rates, location of fuel injector/ignitor as well as the aerodynamics in the area of ignition. This is particularly true for Dry Low Emissions (DLE) systems as there are pilot/main flow rates to balance as well.
The freedom in positioning of an ignitor to achieve reliable ignition of a liquid pilot fuel is very limited. The ignitor and a respective pilot fuel injector are separate items and rely on a local aerodynamics inside the burner volume of the combustor to wash the injected pilot fuel spray over the ignitor.
The fluid flow in the burner volume inside the combustion chamber is relied upon to wash the injected pilot fuel over the ignitor. Therefore, if the local fluid flow tends to move away from the ignitor surface, then the fuel could be swept away from the ignitor without being ignited.
In particular, the aerodynamics within the combustor is designed to reach an optimum at full load running where fuel wash over any parts of the combustor is undesirable as it can cause build of coke. As a consequence, the aerodynamics at the ignitor is poor in a start and lightning phase of the gas turbine.
In order to improve ignition reliability, several methods have been employed such as gas assisted ignition, plasma ignitors and higher power ignitors. Gas assisted ignition requires a further gas supply, e.g. from gas bottles, which can be costly to maintain. Plasma ignitors currently have short lifetimes and there is no running experience of using them in a gas turbine. Higher power ignitors burn out quickly and the service life is short.
FIG. 9 shows a conventional pilot burner device, which comprises a conventional pilot body 900. A conventional pilot surface 901 of the conventional pilot body 900 facing an inner volume (burner volume) of the conventional combustion chamber. A conventional fuel injector 902 and a conventional ignitor unit 903 is installed within the conventional pilot body 900, such that a conventional fuel spray 904 is injectable into the inner volume. A flow direction 106 of fluids in the inner volume directs the injected conventional fuel spray 904 to the conventional ignitor unit 903.
EP 0 728 989 B1 discloses a gas turbine engine combustor. Fuel is injected by a pilot burner and is directed by lips in the circumference of the pilot burner radially to a centre of an inner volume of the combustion chamber. Columns of air which are injected from a base area of the pilot burner directs the injected fuel away from the pilot burner surface.
WO 2009/056425 discloses a combustor for a gas turbine engine which comprises a burner head. Fuel is injected by a pilot fuel nozzle which is positioned off-center with respect to a longitudinal axis of the burner head. A fuel is injected by the nozzle with a predefined cone-shaped spray direction.
EP 1 837 597 A2 discloses a burner for a combustion chamber which comprises a pilot body. A spray nozzle is attached to a burner, wherein the spray nozzles spray fuel into an inner volume of the combustion chamber. A plurality of blowing ports injects air for combustion into a mixing portion of the inner volume of the combustion chamber and along a direction away from a burner surface.
EP 1 279 897 A2 discloses a pilot nozzle of a gas turbine combustor. The pilot nozzle is arranged near a main nozzle of a combustor which injects fuel. Furthermore, pilot air is injected into the gas turbine combustor which directs the injected fuel away from the pilot burner surface of the pilot burner.